As the use of flexible substrates for membrane switch assemblies, flexible circuit panel assemblies and the like increases, there is a concomitant demand for screen printable dielectric coating compositions that are compatible with the flexible substrates.
As used herein the term "screen printable" means that a screen is imaged and the coating is applied only to the desired area rather than completely covering the surface of the substrate. Dielectric coatings used for ceramic components and rigid circuit panel assemblies are generally unsuitable because of high curing temperatures and because they generally produce brittle coatings. These coatings, therefore, are incompatible with flexible substrates. The flexible substrate deteriorates at the curing temperature and the coatings crack when the substrate is flexed, bent or folded.
Current means for meeting industries' needs include the use of additional layers of film which are laminated over the area to be insulated. The use of additional layers of film, however, increases the cost of the product, the number of manufacturing processing steps and the type of manufacturing equipment needed.
Various problems are associated with the formulation of dielectric coatings particularly for substrates having polymeric conductors, and/or conductive ink circuitry. Use of a solvent based coating composition may cause underlying polymeric conductors to deteriorate. Epoxy based systems generally produce coatings that are not sufficiently compliant for use on flexible substrates. In addition they often require curing temperatures that are detrimental to flexible substrates.
It is an object of this invention to provide a dielectric coating composition that is compatible with commerical conductive inks, present manufacturing processes and existing equipment.
Futhermore, it is an object of this invention to provide a dielectric coating that will be compliant and will not crack when the underlying substrate is flexed, bent or folded, and in addition will be sufficiently compliant to accommodate relative movement between circuit members and substrate caused by different coefficients of expansion. It is also an object of the invention to provide a coating that will adhere well to a variety of substrates including flexible substrates such as polyesters, polyimides, polycarbonates, as well as more rigid substrates including phenolics, fiberglass and metals such as aluminum.
These and other objects and advantages of the present invention are achieved by the dielectric coating composition disclosed herein.
The coating composition is comprised of a homogeneous mixture of from about 10 percent to about 50 percent, preferably 20 percent to 35 percent of an unsaturated thiolene prepolymer system; from about 0 percent to about 35 percent, preferably 20 percent to 25 percent of an acrylated prepolymer system, from about 10 percent to about 40 percent, preferably 15 percent to 25 percent of at least one crosslinking diluent monomer and from about 4 percent to about 18 percent, preferably 7.5 percent to 12 percent of at least one plasticizer. From about 3 percent to about 12 percent, preferably 5 percent to 10 percent of at least one nonshrinking monomer may be added to control the shrinkage of the cured coating. Fillers, photoinitiators, catalysts, adhesion promotors and flow agents may also be added depending upon the method used to apply and polymerize the coating layer.
The coating composition in the preferred embodiment is screen printable using techniques known in the art and is radiation curable. The dielectric coating composition is applied selectively to insulate the conductor members in the desired areas on the substrate using screen printing techniques as known in the industry. The coating is cured by ultraviolet light. The compliance of the cured dielectric layer accommodates the limited relative movement between the conductors and the substrate resulting from different coefficients of thermal expansion or from flexure of the substrate.
Some of the objects, advantages and uses of the invention having been stated, others will appear as the description proceeds, when taken in conjunction with the accompanying drawings.